Myocardial infarction due to atherosclerosis of coronary arteries remains the leading cause of death in the United States. It has become clear that changes of cellular/systemic redox state, resulting in increases in inflammation (e.g., TNF) and reactive oxygen species (ROS), represent a common pathogenic mechanism for atherosclerosis. The vascular cell that primarily limits the inflammatory and atherosclerotic process is the endothelial cell (EC). ROS-induced reduction in NO bioavailability and increase of EC apoptosis results in a proatherogenic state. Increasing evidence supports that ROS generated from mitochondria in vasculature significantly contribute to EC dysfunction and atherosclerotic progression. Furthermore, recent data suggest that mitochondria normally produce the strongest reducing environment among all cellular organelles, and mitochondria are especially vulnerable to oxidation in response to stress stimuli including proinflammatory cytokines. A key system regulating mitochondria redox is mitochondria-specific thioredoxin (Trx2) system, consisting of Trx2, Trx2 reductase (TrxR2) and Trx2-depndent peroxidase (Prx3). Little is known for the role of mitochondrial Trx2 system in vasculature. Our data suggest that mitochondrial Trx2 may play critical roles in maintaining mitochondria reduced state, preventing ROS-induced EC dysfunction. Specifically, we have used both EC-specific transgenic and knockout mice, and demonstrated a critical role of Trx2 in regulating endothelium functions by increasing NO bioactivity. We also show that Trx2 inhibits the activity of proapoptotic protein kinase ASK1 through protein-protein interactions, protecting EC from TNF and ROS- induced apoptosis. We propose that Trx2 prevents ROS-induced EC dysfunction through two distinct and cooperative pathways: Trx2 maintains a reduced state of mitochondria in EC, reducing ROS generation leading and increasing NO bioactivity;Trx2 protects ROS-induced EC apoptosis by directly binding to ASK1. We further hypothesize that increased NO bioactivity and decreased apoptotic responses prevent EC dysfunction and atherosclerotic development. To explore these hypotheses, we propose the following specific aims: 1) Determine the mechanisms by which Trx2 preserves NO bioactivity and EC function. 2) Determine the mechanisms by which Trx2 inhibits mitochondrial ASK1-mediated apoptosis. 3) Determine the role of Trx2 in preventing EC dysfunction and atherosclerosis development/progression in a mouse model. This proposal uses both in vitro and in vivo models to determine the roles of Trx2 in protection against ROS- induced EC dysfunction and atherosclerosis development/progression. These studies, if successful, will facilitate the development of new therapeutic approaches to control atherosclerosis progression and myocardial infarction. Project Narrative: Myocardial infarction due to narrowing of arteries manifesting as decreased blood flow remains the leading cause of death in the United States. We will study the effects of a antioxidant protein thioredoxin, on vascular endothelium. Our work may lead to better tests and treatments for atherosclerosis patients.